Setting retarder

Abstract

Gypsum plaster having a delayed setting time is made by adding a mixture of mesotartaric acid and l-tartaric acid or d,l-tartaric acid or a mixture of l-tartaric acid and d,l-tartaric acid to the gypsum plaster.

What is claimed is: 
     
       1. Gypsum plaster having a mixture of tartaric acids therein to delay the setting time of the plaster, said tartaric acids consisting of a mixture of meso-tartaric acid with ( 1) l-tartaric acid, (2) d,l-tartaric acid or (3) both of l-tartaric acid and d,l-tartaric acid, said mixture of tartaric acids containing 5 to 80% of meso-tartaric acid by weight, said plaster containing 0.01 to 0.5% by weight of said mixture of tartaric acids, and when said mixture of tartaric acids is (2) or (3) the plaster has sufficient calcium hydroxide that a 5% aqueous paste of the plaster has a pH of at least 11.5. 
     
     
       2. Gypsum plaster according to claim 1 wherein said mixture of tartaric acids contains 20 to 60% of meso-tartatic acid. 
     
     
       3. Gypsum plaster according to claim 1 wherein the plaster contains 0.015 to 0.2% of said mixture of tartaric acids. 
     
     
       4. Gypsum plaster according to claim 3 wherein the plaster has sufficient calcium hydroxide that a 5% aqueous paste of the plaster has a pH of 12 up to the pH of saturated calcium hydroxide. 
     
     
       5. Gypsum plaster according to claim 1 wherein the plaster has sufficient calcium hydroxide that a 5% aqueous paste of the plaster has a pH of 12 up to the pH of saturated calcium hydroxide. 
     
     
       6. Gypsum plaster according to claim 1 wherein the plaster has sufficient calcium hydroxide that a 5% aqueous paste of the plaster has a pH of at least 11.5. 
     
     
       7. Gypsum plaster according to claim 6 wherein the mixture of tartaric acids consists of a mixture of meso-tartaric acid with either (1) d,l-tartaric acid or (2) both l-tartaric acid and d,l-tartaric acid. 
     
     
       8. Gypsum plaster according to claim 1 wherein the mixture of tartaric acids consists of a mixture of meso-tartaric acid and l-tartaric acid.